The invention is based on a friction-wheel epicyclic gear with bevel wheels. One such friction-wheel epicyclic gear, known for instance from German Patent Disclosure DE196 44 133 A1, has bevel wheels which roll on a friction wheel, and the contact pressure between the bevel wheels and the friction wheel is variable. For varying the contact pressure, this reference proposes that an rpm- or moment-dependent regulator be provided. That is, a corresponding contact pressure is associated with a certain rpm or a certain torque. So that no slip will occur, the contact pressure must be set higher than absolutely necessary. The result, however, is increased wear of the bevel wheels and/or the friction wheel. Conversely, if a certain amount of slip is tolerated, then this causes increased power losses or poorer efficiency of the friction-wheel epicyclic gear.
The friction-wheel epicyclic gear of the invention has the epicyclic gear can be driven at the slip limit. As a result, the friction-wheel epicyclic gear of the invention has less wear and at the same time has high efficiency. This is because the point of departure of varying the contact pressure of the bevel wheels at the friction wheel is not the rotary speed of the torque, but the slip.
One possibility for this is for a bevel wheel to serve not to transmit power but rather, via a pinion, to drive a shaft. By detecting and comparing the rotary speed of this shaft with the rotary speed of the shaft that is in the line of force, the slip can easily be detected and as a result the contact pressure can be varied.
A second possibility for detecting the slip is to detect the input rpm and the output rpm of the gear and to assign a value that corresponds to the output rpm of the friction-wheel epicyclic gear without slip to the applicable input rpm. The contact pressure can then be varied as a function of the difference between the output rpm and that value.